Aside from mechanical requirements, you should consider the part’s operating environment. Will the part be exposed to water, chemicals or high or low temperatures? Does it need to be fire resistant? Again, product density plays a role. Generally, higher density foams have better corrosion and temperature resistance than lower density materials of the same polymer. The sheet configuration can also affect performance as well as the lamination process.
Throughout the selection process, keep in mind that some aspects of a part’s behavior are not exclusively attributed to the core material. For example, changing the resin may vastly improve fire resistance.
Another critical consideration is the lamination process. Important features to reflect upon are resin compatibility (styrene resistance), temperature resistance (dimensional stability) and open cell content. For certain foams, resistance to styrene is generally dependent on density. (Lower density is less resistant.) Also keep in mind that foams not designed to operate at high temperature are fully capable of withstanding the heat from resin exotherm, so don’t mistakenly eliminate materials because you fear that the resin will get hot: there is a difference between short-term temperature spikes from exotherm or post curing and long-term exposure with an elevated temperature cure.
Once the material is selected, the manufacturing process and geometry of your part will determine the core’s configuration or finish. Here’s where proper core installation comes into play. Often bad performance is blamed on the core material, when poor quality laminate was the cause. Always follow these rules:
- Avoid “never bonds.”
- Fill the kerfs.
- Fillet all core edges.
- Segregate the core from all openings.
This brings us to total cost, which is a combination of the core material, finishing and resin absorption. Let’s take a look at all three:
Material: Look for value. Often the lowest cost product will not produce the least expensive part. It is quite easy to determine which products produce the best “bang for your buck.”
Finishing: Unless you are building flat panels in a press, all core materials (besides honeycomb) should be perforated or otherwise scored to allow air and other gasses to escape from under the core. Sheets must also be cut to conform to curved surfaces. Certain foams may be thermoformed to produce lighter parts. Grooves may be added to promote resin flow.
Resin absorption: Balance conformability with resin uptake. Treatments are also available to reduce the amount of resin absorbed by the core when infused.
Overall, the proper selection of a sandwich core material may seem like an arduous task, but many questions answer themselves fairly quickly. If you apply the “right tool for the right job” maxim, then sandwich core materials will help you build better parts with greater quality and improve your bottom line.
